This invention relates to surface coated tungsten carbide-base sintered hard alloy materials for inserts of cutting tools (hereinafter abbreviated as coated cutting tool materials), of which surfaces are coated with a hard layer, and more particularly to cutting tool materials of this kind which exhibit very excellent cutting performance by virtue of excellent wear resistance and toughness thereof, when used not only in continuous cutting of steel but also in intermittent cutting of same.
Conventionally, there have been widely employed coated cutting tool materials formed of tungsten carbide(WC)-base sintered hard alloys. Coated cutting tool materials of this kind are typically produced by preparing as starting powders (Ti, W)C powder, TiC powder, NbC powder, TaC powder, (Ti, Nb)C powder, WC powder, and Co powder, etc., then blending them into a predetermined composition, kneading the blended powder into a mixed powder, compressing the mixed powder into a green compact, and sintering the green compact for 1-2 hours in a vacuum atmosphere under a pressure of 10.sup.-1 torr or less and at a temperature within a range from 1300.degree. to 1500.degree. C. The resulting cutting tool materials have substrates consisting essentially of: 5-30 percent by weight (hereinafter percentages are weight percentages throughout the whole specification) one composite metal carbide, as a component forming a hard disperse phase, selected from the group consisting of (Ti, W)C, (Ti, Nb, W)C, (Ti, Ta, W)C, and (Ti, Nb, Ta, W)C; 4-10 percent Co as a component forming a binder phase; and the balance of WC as another component forming the hard disperse phase and inevitable impurities. Each substrate is coated, by an ordinary coating method such as a chemical vapor deposition method or a physical vapor deposition method, with a hard coating layer composed of an inner layer consisting essentially of TiC and having a mean layer thickness of 2-8 microns, and an outer layer formed by a single layer of one component or a double layer of two compounds selected from the group consisting of TiCO, TiCNO, and Al.sub.2 O.sub.3 and having a mean layer thickness of 0.3-3 microns.
Although cutting tools formed of the above conventional coated cutting tool materials exhibit excellent wear resistance when used in continuous cutting of steel, their cutting edges are liable to be chipped when used in intermittent cutting of steel due to insufficient toughness of their substrates, and hence the conventional cutting tools have rather short lives.